1. Field of the Invention
The present invention relates to a process for producing a halftone mask, more particularly, to a process for producing a halftone mask comprising a transparent substrate, a translucent film and a light-block film. The process enables an easy and secure patterning of the translucent film.
2. Description of Related Art
Generally, a halftone mask have a film of molybdenum silicide as a translucent film which provides halftone and a film of chromium as a light-block film which are laminated in desired patterns on a transparent substrate typically made of quartz.
Where a halftone mask as described above is to be used as a mask for a KrF excimer exposure, the molybdenum silicide film and the chromium film are formed to be about 93 nm and about 70 nm thick, respectively.
Such a halftone mask is conventionally produced by the following process in association with FIGS. 2(a) to 2(g).
As shown in FIG. 2(a), a film 22 of molybdenum silicide having a thickness of 93 nm and a film 23 of chromium having a thickness of 70 nm are sequentially formed on a substrate 21 of quartz by a vacuum deposition method or a sputtering method. Further an electron beam (EB) resist film 24 is formed thereon to be about 450 to about 550 nm thick by a spin-on coating method. An electron beam is irradiated to the resulting resist film 24. This electron beam irradiation is performed separately on a first write area 24a where the resist film will remain in a predetermined thickness by developed and on a second write area 24b where the resist film will be completely removed by being developed, for the purpose of enhancing the alignment of a pattern on the light-block film and a pattern on the translucent film and minimizing the number of steps in EB writing.
Subsequently, as shown in FIG. 2(b), the EB-irradiated resist film 24 is developed to form a resist film 24 patterned to have the first write area 24a and the second write area 24b.
Thereafter, as shown in FIG. 2(c), the exposed chromium film 23 is dry-etched with a CCl.sub.4 +O.sub.2 gas plasma using the resist film 24 as a mask.
Then, as shown in FIG. 2(d), the exposed molybdenum silicide film 22 is dry-etched with a CF.sub.4 +O.sub.2 +N.sub.2 gas plasma.
Next, as shown in FIG. 2(e), the resist film 24 is ashed with an O.sub.2 plasma to remove the resist film 24 in the first write area 24a.
Subsequently, as shown in FIG. 2(f), the exposed chromium film 23 is wet-etched with a mixed liquid of ammonium cerium (IV) nitrate and perchloric acid using the resulting resist film 24 as a mask.
Lastly, as shown in FIG. 2(g), the remaining resist film 24 is removed by being immersed in dimethylformamide, acetone, and an aqueous solution of sulfuric acid and hydrogen peroxide sequentially.
However, in the above-described process for producing a halftone mask, the ashing rate is not stable when the resist film 24 is ashed with O.sub.2, and therefore, the resist film 24 may sometimes remain in the first write area 24a. The reason why the ashing rate is unstable is that the resistance of the resist film 24 to ashing becomes high due to the dry etching of the molybdenum silicide film 22 with the CF.sub.4 +O.sub.2 +N.sub.2 gas plasma before the resist film 24 is ashed and thereby the ashing rate at the O.sub.2 plasma ashing decreases.
This defective patterning of the resist film 24 by ashing brings about an inaccurate patterning of the molybdenum silicide film 22 in a later step. Thus there is a problem that the production yield of halftone masks is considerably low.